Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, an X-ray imaging apparatus, and a recording medium storing an image processing program.
Description of the Related Art
In recent years, aging is rapidly advancing in Japan. With the aging, the number of patients of osteoarthritis and articular rheumatism is also increasing. There is a magnetic resonance imaging (MRI) apparatus as a medical apparatus that appropriately understands conditions of the articular cartilage. However, the MRI apparatus is expensive, and imaging cost is thus also high. Further, since imaging time of the MRI apparatus is long, the load on a patient is large.
Thus, an X-ray imaging apparatus is used in image diagnosis for the above articular diseases. A common X-ray imaging apparatus captures an image of a subject on the basis of an X-ray absorption coefficient. Since the articular cartilage has a low X-ray absorption coefficient, the articular cartilage is less likely to be reflected on an X-ray image based on the absorption coefficient. On the other hand, since a bone part has a high X-ray absorption coefficient, the bone part is likely to be reflected on the X-ray image. Thus, in the X-ray image, a signal is interrupted in the boundary between bones. Thus, when a conventional X-ray imaging apparatus is used, the thickness of the articular cartilage is merely analogically understood through the distance between two bones adjacent to the articular cartilage (a part in which a signal is interrupted) in the X-ray image.
The application of a Talbot interferometer and a Talbot-Lau interferometer to a medical image is under consideration as a technique taking the place of such a conventional X-ray imaging apparatus.
The Talbot interferometer and the Talbot-Lau interferometer use a technique that does not acquire an X-ray image of a subject on the basis of an X-ray absorption coefficient as performed by a conventional X-ray imaging apparatus, but acquires a phase image of a subject on the basis of an X-ray phase change. The Talbot interferometer and the Talbot-Lau interferometer enable visualization of a living body soft tissue that cannot be read by a conventional X-ray imaging apparatus.
However, a differential phase image (hereinbelow, also referred to as “differential image”) that can be obtained by the Talbot interferometer and the Talbot-Lau interferometer largely differs in view from an X-ray image of a common X-ray imaging apparatus, and is not familiar to doctors. Thus, image processing is performed on the differential image to make the differential image similar to an absorption image familiar to doctors. Examples of citations that disclose such image processing include US 2013/0156284 A, US 2013/0279659 A, and US 2014/0177790 A.
In US 2013/0156284 A, an optimization operation is used on a differential image to generate a phase image. In the optimization operation, an X-direction objective function is performed so that the gradient of a phase and a differential phase becomes minimum, and a Y-direction objective function is performed so that the sum of gradients with an upper or a lower pixel becomes minimum.
In US 2013/0279659 A, secondary-norm constrained optimization is used to generate a phase image. In the optimization operation, an X-direction objective function is determined by the secondary norm of the gradient of a phase and a differential phase, and a Y-direction objective function is reconfigured by optimization to which the secondary norm of the gradient with an upper or lower pixel value of a target pixel value is added.
The imaging apparatus disclosed in US 2014/0177790 A moves a two-dimensional grating in two directions to capture an image of a subject so that a phase image is reconfigured with respect to differential phase data holding information in the two directions.
When a diffraction grating is moved in one direction (X direction) to obtain a differential image as performed by the Talbot interferometer, the differential image includes only differential information in the direction in which the diffraction grating is moved, and includes no differential information in a direction other than the X direction. Thus, when the phase image is calculated on the basis of the differential image, many false images are included.
Regarding this point, in both the image processing of US 2013/0156284 A and the image processing of US 2013/0279659 A, the optimization is performed on the assumption that differential information in a direction different from the movement direction of the diffraction grating is continuous, and there is substantially no difference between signal values of pixels adjacent to a target pixel in the up-down direction (Y direction). Such an optimization operation merely calculates an average of differential values of pixels that are continuous in the up-down direction, and blurs the differential value on the image.
Further, the two-dimensional grating disclosed in US 2014/0177790 A makes the apparatus configuration, the imaging method, and the configuration of the grating itself more complicated than a one-dimensional grating. Thus, the cost becomes higher than an apparatus that uses a one-dimensional grating.